On my 504, I have a glass 4x5 sandwich (as well as the standard metal holders for the various formats) so I can clamp my negatives in the carrier whichever way I wish, with the only barrier being the back edge of the carrier.
The Ilford Multigrade 500 system fits many enlargers - however, each enlarger needs a dedicated adapter (and, for some enlargers, such as Beseler, dedicated mixing boxes).
In practice, the best solution is to buy a working system (enlarger + multigrade head)
I would second the recommendation of buying an enlarger with the 500 system already installed. One warning though - make sure you get the mixing boxes and the plates that the boxes rest on with the enlarger/ilford head (it is not uncommon to see ilford 500 equipped enlargers sold w/ the mixing boxes missing - they tend to get lost in the shuffle when these systems change hands...and tracking them down can be a royal PITA). In the States, Omega D/ilford 500 setups seem to be the most popular ilford 500 variants by far (something to keep in mind if you need to track down mounts, boxes, etc.) I would imagine that in your location Dursts and DeVeres would be the most common variants. In any case, you made a good choice...the Ilford 500 system is a joy to use (and enough of them were sold to ensure that 2nd hand parts/donors will be available for quite awhile)
About the OP's question: The advice of finding a complete enlarger with an already fitted 500H is very good advice. As the OP lives in Holland, I recon that Durst, Leitz, Agfa and possbily DeVere are more common than the american enlargers (Beseler, Omega ...).